The combustion of fossil fuel produces a flue gas with dilute SO2 and CO2 which are difficult to remove in a single removal process. The incumbent technology for their removal is to remove SO2 with a flue gas desulfurization (FGD) process and then remove CO2 with a solvent scrubbing. The FGD process requires the use of a reagent such as limestone slurry, and it generates a waste product such as calcium sulfate that needs to be disposed offsite. The current technology for capturing CO2 from the flue gas is by solvent scrubbing. Many solvents are under development, including amines that are tolerant to the impurities in coal fired boiler flue gas as well as low in solvent regeneration energy. Current solvent technology requires substantial amount of steam to regenerate the spent solvent for recycle, and this can reduce the power plant output by as much as 25%.
Another method to capture CO2 from flue gas is the use of oxyfuel combustion. In an oxyfuel combustion boiler, fossil fuel (coal, oil or gas) is combusted using a high purity oxygen, generally 95% oxygen or higher. This method of combustion reduces the amount of nitrogen in the flue gas, thus facilitating carbon dioxide capture. The flue gas volume from an oxy combustion boiler is about only 25% that of an air fired boiler of the same size. The flue gas is first cooled to recover moisture in the flue gas. The dried oxyfuel flue gas contains about 90% of CO2 compared to about 14% for an air-fired boiler flue gas. The use of oxyfuel combustion eliminates the need for a CO2 concentrating process such as an amine plant.
The generic oxy-combustion process under development by the power industry is generally comprised of a wet flue gas desulfurization (FGD) unit for SO2 capture followed by a CO2 purification unit (CPU). A portion of desulfurized gas is re-circulated back to the boiler as a modulating gas to quench the furnace temperature due to oxygen firing. The CPU is comprised a cryogenic type cooling and separation of CO2 from oxygen and nitrogen gases which were infiltrated into the flue gas due to air leakage in the downstream emission control equipment.
This invention provides a method to remove SO2 and CO2 in a combined process rather than remove them using two separate processes. In addition, with this invention, the SO2 removed is a liquid product that can be transported in vessels and is a saleable product. The CO2 product purity can be controlled to meet the final product applications. The combined SO2 and CO2 removal process generates no desulfurization waste product, and offers a lower capital cost option for carbon capture. This process is superior to the combined SOx and CO2 removal process described in U.S. Pat. No. 7,416,716 where SO2 is removed as sulfuric acid rather than a liquid product. It is also superior to the process described in U.S. Pat. No. 6,737,031 where calcium based reagent is needed to capture SO2.